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#[cfg(all(test, not(target_os = "emscripten")))]
mod tests;
use crate::cell::UnsafeCell;
use crate::fmt;
use crate::ops::{Deref, DerefMut};
use crate::ptr::NonNull;
use crate::sync::{poison, LockResult, TryLockError, TryLockResult};
use crate::sys_common::rwlock as sys;
/// A reader-writer lock
///
/// This type of lock allows a number of readers or at most one writer at any
/// point in time. The write portion of this lock typically allows modification
/// of the underlying data (exclusive access) and the read portion of this lock
/// typically allows for read-only access (shared access).
///
/// In comparison, a [`Mutex`] does not distinguish between readers or writers
/// that acquire the lock, therefore blocking any threads waiting for the lock to
/// become available. An `RwLock` will allow any number of readers to acquire the
/// lock as long as a writer is not holding the lock.
///
/// The priority policy of the lock is dependent on the underlying operating
/// system's implementation, and this type does not guarantee that any
/// particular policy will be used. In particular, a writer which is waiting to
/// acquire the lock in `write` might or might not block concurrent calls to
/// `read`, e.g.:
///
/// <details><summary>Potential deadlock example</summary>
///
/// ```text
/// // Thread 1 | // Thread 2
/// let _rg = lock.read(); |
/// | // will block
/// | let _wg = lock.write();
/// // may deadlock |
/// let _rg = lock.read(); |
/// ```
/// </details>
///
/// The type parameter `T` represents the data that this lock protects. It is
/// required that `T` satisfies [`Send`] to be shared across threads and
/// [`Sync`] to allow concurrent access through readers. The RAII guards
/// returned from the locking methods implement [`Deref`] (and [`DerefMut`]
/// for the `write` methods) to allow access to the content of the lock.
///
/// # Poisoning
///
/// An `RwLock`, like [`Mutex`], will become poisoned on a panic. Note, however,
/// that an `RwLock` may only be poisoned if a panic occurs while it is locked
/// exclusively (write mode). If a panic occurs in any reader, then the lock
/// will not be poisoned.
///
/// # Examples
///
/// ```
/// use std::sync::RwLock;
///
/// let lock = RwLock::new(5);
///
/// // many reader locks can be held at once
/// {
/// let r1 = lock.read().unwrap();
/// let r2 = lock.read().unwrap();
/// assert_eq!(*r1, 5);
/// assert_eq!(*r2, 5);
/// } // read locks are dropped at this point
///
/// // only one write lock may be held, however
/// {
/// let mut w = lock.write().unwrap();
/// *w += 1;
/// assert_eq!(*w, 6);
/// } // write lock is dropped here
/// ```
///
/// [`Mutex`]: super::Mutex
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "RwLock")]
pub struct RwLock<T: ?Sized> {
inner: sys::MovableRwLock,
poison: poison::Flag,
data: UnsafeCell<T>,
}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
/// RAII structure used to release the shared read access of a lock when
/// dropped.
///
/// This structure is created by the [`read`] and [`try_read`] methods on
/// [`RwLock`].
///
/// [`read`]: RwLock::read
/// [`try_read`]: RwLock::try_read
#[must_use = "if unused the RwLock will immediately unlock"]
#[must_not_suspend = "holding a RwLockReadGuard across suspend \
points can cause deadlocks, delays, \
and cause Futures to not implement `Send`"]
#[stable(feature = "rust1", since = "1.0.0")]
#[clippy::has_significant_drop]
#[cfg_attr(not(test), rustc_diagnostic_item = "RwLockReadGuard")]
pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
// NB: we use a pointer instead of `&'a T` to avoid `noalias` violations, because a
// `Ref` argument doesn't hold immutability for its whole scope, only until it drops.
// `NonNull` is also covariant over `T`, just like we would have with `&T`. `NonNull`
// is preferable over `const* T` to allow for niche optimization.
data: NonNull<T>,
inner_lock: &'a sys::MovableRwLock,
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Send for RwLockReadGuard<'_, T> {}
#[stable(feature = "rwlock_guard_sync", since = "1.23.0")]
unsafe impl<T: ?Sized + Sync> Sync for RwLockReadGuard<'_, T> {}
/// RAII structure used to release the exclusive write access of a lock when
/// dropped.
///
/// This structure is created by the [`write`] and [`try_write`] methods
/// on [`RwLock`].
///
/// [`write`]: RwLock::write
/// [`try_write`]: RwLock::try_write
#[must_use = "if unused the RwLock will immediately unlock"]
#[must_not_suspend = "holding a RwLockWriteGuard across suspend \
points can cause deadlocks, delays, \
and cause Future's to not implement `Send`"]
#[stable(feature = "rust1", since = "1.0.0")]
#[clippy::has_significant_drop]
#[cfg_attr(not(test), rustc_diagnostic_item = "RwLockWriteGuard")]
pub struct RwLockWriteGuard<'a, T: ?Sized + 'a> {
lock: &'a RwLock<T>,
poison: poison::Guard,
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Send for RwLockWriteGuard<'_, T> {}
#[stable(feature = "rwlock_guard_sync", since = "1.23.0")]
unsafe impl<T: ?Sized + Sync> Sync for RwLockWriteGuard<'_, T> {}
impl<T> RwLock<T> {
/// Creates a new instance of an `RwLock<T>` which is unlocked.
///
/// # Examples
///
/// ```
/// use std::sync::RwLock;
///
/// let lock = RwLock::new(5);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
#[inline]
pub const fn new(t: T) -> RwLock<T> {
RwLock {
inner: sys::MovableRwLock::new(),
poison: poison::Flag::new(),
data: UnsafeCell::new(t),
}
}
}
impl<T: ?Sized> RwLock<T> {
/// Locks this rwlock with shared read access, blocking the current thread
/// until it can be acquired.
///
/// The calling thread will be blocked until there are no more writers which
/// hold the lock. There may be other readers currently inside the lock when
/// this method returns. This method does not provide any guarantees with
/// respect to the ordering of whether contentious readers or writers will
/// acquire the lock first.
///
/// Returns an RAII guard which will release this thread's shared access
/// once it is dropped.
///
/// # Errors
///
/// This function will return an error if the RwLock is poisoned. An RwLock
/// is poisoned whenever a writer panics while holding an exclusive lock.
/// The failure will occur immediately after the lock has been acquired.
///
/// # Panics
///
/// This function might panic when called if the lock is already held by the current thread.
///
/// # Examples
///
/// ```
/// use std::sync::{Arc, RwLock};
/// use std::thread;
///
/// let lock = Arc::new(RwLock::new(1));
/// let c_lock = Arc::clone(&lock);
///
/// let n = lock.read().unwrap();
/// assert_eq!(*n, 1);
///
/// thread::spawn(move || {
/// let r = c_lock.read();
/// assert!(r.is_ok());
/// }).join().unwrap();
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn read(&self) -> LockResult<RwLockReadGuard<'_, T>> {
unsafe {
self.inner.read();
RwLockReadGuard::new(self)
}
}
/// Attempts to acquire this rwlock with shared read access.
///
/// If the access could not be granted at this time, then `Err` is returned.
/// Otherwise, an RAII guard is returned which will release the shared access
/// when it is dropped.
///
/// This function does not block.
///
/// This function does not provide any guarantees with respect to the ordering
/// of whether contentious readers or writers will acquire the lock first.
///
/// # Errors
///
/// This function will return the [`Poisoned`] error if the RwLock is poisoned.
/// An RwLock is poisoned whenever a writer panics while holding an exclusive
/// lock. `Poisoned` will only be returned if the lock would have otherwise been
/// acquired.
///
/// This function will return the [`WouldBlock`] error if the RwLock could not
/// be acquired because it was already locked exclusively.
///
/// [`Poisoned`]: TryLockError::Poisoned
/// [`WouldBlock`]: TryLockError::WouldBlock
///
/// # Examples
///
/// ```
/// use std::sync::RwLock;
///
/// let lock = RwLock::new(1);
///
/// match lock.try_read() {
/// Ok(n) => assert_eq!(*n, 1),
/// Err(_) => unreachable!(),
/// };
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn try_read(&self) -> TryLockResult<RwLockReadGuard<'_, T>> {
unsafe {
if self.inner.try_read() {
Ok(RwLockReadGuard::new(self)?)
} else {
Err(TryLockError::WouldBlock)
}
}
}
/// Locks this rwlock with exclusive write access, blocking the current
/// thread until it can be acquired.
///
/// This function will not return while other writers or other readers
/// currently have access to the lock.
///
/// Returns an RAII guard which will drop the write access of this rwlock
/// when dropped.
///
/// # Errors
///
/// This function will return an error if the RwLock is poisoned. An RwLock
/// is poisoned whenever a writer panics while holding an exclusive lock.
/// An error will be returned when the lock is acquired.
///
/// # Panics
///
/// This function might panic when called if the lock is already held by the current thread.
///
/// # Examples
///
/// ```
/// use std::sync::RwLock;
///
/// let lock = RwLock::new(1);
///
/// let mut n = lock.write().unwrap();
/// *n = 2;
///
/// assert!(lock.try_read().is_err());
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn write(&self) -> LockResult<RwLockWriteGuard<'_, T>> {
unsafe {
self.inner.write();
RwLockWriteGuard::new(self)
}
}
/// Attempts to lock this rwlock with exclusive write access.
///
/// If the lock could not be acquired at this time, then `Err` is returned.
/// Otherwise, an RAII guard is returned which will release the lock when
/// it is dropped.
///
/// This function does not block.
///
/// This function does not provide any guarantees with respect to the ordering
/// of whether contentious readers or writers will acquire the lock first.
///
/// # Errors
///
/// This function will return the [`Poisoned`] error if the RwLock is
/// poisoned. An RwLock is poisoned whenever a writer panics while holding
/// an exclusive lock. `Poisoned` will only be returned if the lock would have
/// otherwise been acquired.
///
/// This function will return the [`WouldBlock`] error if the RwLock could not
/// be acquired because it was already locked exclusively.
///
/// [`Poisoned`]: TryLockError::Poisoned
/// [`WouldBlock`]: TryLockError::WouldBlock
///
///
/// # Examples
///
/// ```
/// use std::sync::RwLock;
///
/// let lock = RwLock::new(1);
///
/// let n = lock.read().unwrap();
/// assert_eq!(*n, 1);
///
/// assert!(lock.try_write().is_err());
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<'_, T>> {
unsafe {
if self.inner.try_write() {
Ok(RwLockWriteGuard::new(self)?)
} else {
Err(TryLockError::WouldBlock)
}
}
}
/// Determines whether the lock is poisoned.
///
/// If another thread is active, the lock can still become poisoned at any
/// time. You should not trust a `false` value for program correctness
/// without additional synchronization.
///
/// # Examples
///
/// ```
/// use std::sync::{Arc, RwLock};
/// use std::thread;
///
/// let lock = Arc::new(RwLock::new(0));
/// let c_lock = Arc::clone(&lock);
///
/// let _ = thread::spawn(move || {
/// let _lock = c_lock.write().unwrap();
/// panic!(); // the lock gets poisoned
/// }).join();
/// assert_eq!(lock.is_poisoned(), true);
/// ```
#[inline]
#[stable(feature = "sync_poison", since = "1.2.0")]
pub fn is_poisoned(&self) -> bool {
self.poison.get()
}
/// Clear the poisoned state from a lock
///
/// If the lock is poisoned, it will remain poisoned until this function is called. This allows
/// recovering from a poisoned state and marking that it has recovered. For example, if the
/// value is overwritten by a known-good value, then the mutex can be marked as un-poisoned. Or
/// possibly, the value could be inspected to determine if it is in a consistent state, and if
/// so the poison is removed.
///
/// # Examples
///
/// ```
/// #![feature(mutex_unpoison)]
///
/// use std::sync::{Arc, RwLock};
/// use std::thread;
///
/// let lock = Arc::new(RwLock::new(0));
/// let c_lock = Arc::clone(&lock);
///
/// let _ = thread::spawn(move || {
/// let _lock = c_lock.write().unwrap();
/// panic!(); // the mutex gets poisoned
/// }).join();
///
/// assert_eq!(lock.is_poisoned(), true);
/// let guard = lock.write().unwrap_or_else(|mut e| {
/// **e.get_mut() = 1;
/// lock.clear_poison();
/// e.into_inner()
/// });
/// assert_eq!(lock.is_poisoned(), false);
/// assert_eq!(*guard, 1);
/// ```
#[inline]
#[unstable(feature = "mutex_unpoison", issue = "96469")]
pub fn clear_poison(&self) {
self.poison.clear();
}
/// Consumes this `RwLock`, returning the underlying data.
///
/// # Errors
///
/// This function will return an error if the RwLock is poisoned. An RwLock
/// is poisoned whenever a writer panics while holding an exclusive lock. An
/// error will only be returned if the lock would have otherwise been
/// acquired.
///
/// # Examples
///
/// ```
/// use std::sync::RwLock;
///
/// let lock = RwLock::new(String::new());
/// {
/// let mut s = lock.write().unwrap();
/// *s = "modified".to_owned();
/// }
/// assert_eq!(lock.into_inner().unwrap(), "modified");
/// ```
#[stable(feature = "rwlock_into_inner", since = "1.6.0")]
pub fn into_inner(self) -> LockResult<T>
where
T: Sized,
{
let data = self.data.into_inner();
poison::map_result(self.poison.borrow(), |()| data)
}
/// Returns a mutable reference to the underlying data.
///
/// Since this call borrows the `RwLock` mutably, no actual locking needs to
/// take place -- the mutable borrow statically guarantees no locks exist.
///
/// # Errors
///
/// This function will return an error if the RwLock is poisoned. An RwLock
/// is poisoned whenever a writer panics while holding an exclusive lock. An
/// error will only be returned if the lock would have otherwise been
/// acquired.
///
/// # Examples
///
/// ```
/// use std::sync::RwLock;
///
/// let mut lock = RwLock::new(0);
/// *lock.get_mut().unwrap() = 10;
/// assert_eq!(*lock.read().unwrap(), 10);
/// ```
#[stable(feature = "rwlock_get_mut", since = "1.6.0")]
pub fn get_mut(&mut self) -> LockResult<&mut T> {
let data = self.data.get_mut();
poison::map_result(self.poison.borrow(), |()| data)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let mut d = f.debug_struct("RwLock");
match self.try_read() {
Ok(guard) => {
d.field("data", &&*guard);
}
Err(TryLockError::Poisoned(err)) => {
d.field("data", &&**err.get_ref());
}
Err(TryLockError::WouldBlock) => {
struct LockedPlaceholder;
impl fmt::Debug for LockedPlaceholder {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.write_str("<locked>")
}
}
d.field("data", &LockedPlaceholder);
}
}
d.field("poisoned", &self.poison.get());
d.finish_non_exhaustive()
}
}
#[stable(feature = "rw_lock_default", since = "1.10.0")]
impl<T: Default> Default for RwLock<T> {
/// Creates a new `RwLock<T>`, with the `Default` value for T.
fn default() -> RwLock<T> {
RwLock::new(Default::default())
}
}
#[stable(feature = "rw_lock_from", since = "1.24.0")]
impl<T> From<T> for RwLock<T> {
/// Creates a new instance of an `RwLock<T>` which is unlocked.
/// This is equivalent to [`RwLock::new`].
fn from(t: T) -> Self {
RwLock::new(t)
}
}
impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
/// Create a new instance of `RwLockReadGuard<T>` from a `RwLock<T>`.
// SAFETY: if and only if `lock.inner.read()` (or `lock.inner.try_read()`) has been
// successfully called from the same thread before instantiating this object.
unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockReadGuard<'rwlock, T>> {
poison::map_result(lock.poison.borrow(), |()| RwLockReadGuard {
data: NonNull::new_unchecked(lock.data.get()),
inner_lock: &lock.inner,
})
}
}
impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
/// Create a new instance of `RwLockWriteGuard<T>` from a `RwLock<T>`.
// SAFETY: if and only if `lock.inner.write()` (or `lock.inner.try_write()`) has been
// successfully called from the same thread before instantiating this object.
unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockWriteGuard<'rwlock, T>> {
poison::map_result(lock.poison.guard(), |guard| RwLockWriteGuard { lock, poison: guard })
}
}
#[stable(feature = "std_debug", since = "1.16.0")]
impl<T: fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
#[stable(feature = "std_debug", since = "1.16.0")]
impl<T: fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for RwLockReadGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
// SAFETY: the conditions of `RwLockGuard::new` were satisfied when created.
unsafe { self.data.as_ref() }
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for RwLockWriteGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
// SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
unsafe { &*self.lock.data.get() }
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> DerefMut for RwLockWriteGuard<'_, T> {
fn deref_mut(&mut self) -> &mut T {
// SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
unsafe { &mut *self.lock.data.get() }
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Drop for RwLockReadGuard<'_, T> {
fn drop(&mut self) {
// SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
unsafe {
self.inner_lock.read_unlock();
}
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Drop for RwLockWriteGuard<'_, T> {
fn drop(&mut self) {
self.lock.poison.done(&self.poison);
// SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
unsafe {
self.lock.inner.write_unlock();
}
}
}